Solar cell, method for manufacturing solar cell, and solar cell module

ABSTRACT

Disclosed is a solar cell which is provided with: a semiconductor substrate having a light-receiving surface and a non-light-receiving surface; a PN junction section formed on the semiconductor substrate; a passivation layer formed on the light-receiving surface and/or the non-light-receiving surface; and power extraction electrodes formed on the light-receiving surface and the non-light-receiving surface. The solar cell is characterized in that the passivation layer includes an aluminum oxide film having a thickness off 40 nm or less. As a result of forming a aluminum oxide film having a predetermined thickness on the surface of the substrate, it is possible to achieve excellent passivation performance and excellent electrical contact between silicon and the electrode by merely firing the conductive paste, which is conventional technology. Furthermore, an annealing step, which has been necessary to achieve the passivation effects of the aluminum oxide film in the past, can be eliminated, thus dramatically reducing costs.

TECHNICAL FIELD

This invention relates to a solar cell featuring high productivity, lowcost, and high efficiency, a method for manufacturing the solar cell,and a solar cell module.

BACKGROUND ART

FIG. 1 schematically illustrates a p-type substrate solar cell as oneexample of prior art solar cells which are generally manufactured on amass-scale using single crystal and polycrystalline silicon substrates.A pn junction 103 is formed by diffusing Group V element such asphosphorus into the light-receiving surface of a semiconductor substrate(silicon substrate) 101 in a high concentration to form a n-type layer102. Dielectric films 104 and 105 having a lower refractive index thansilicon are formed on both the major surfaces (light-receiving andnon-light-receiving surfaces) of p or n-type silicon substrate,respectively, for more efficient containment of light. In thesedielectric films 104 and 105, titanium oxide, silicon nitride, siliconcarbide, silicon oxide, tin oxide and the like are widely used. Whilethe thickness of a dielectric film that provides for effective opticalconfinement varies with its refractive index, the thickness of a siliconnitride film, for example, is generally about 80 to 100 nm on thelight-receiving surface and about 90 to 300 nm on the back surface.

Also, on the light-receiving surface and the non-light-receiving (back)surface, electrodes 106 and 107 are formed for extracting photo-createdcarriers. Among methods of forming such electrodes, one method which iswidely used from the aspect of cost is by mixing metal fine particlessuch as silver or aluminum with an organic binder, printing the metalpaste using a screen or the like, and heat treating the paste forbringing it in contact with the substrate. Electrode formation isgenerally preceded by formation of dielectric film. Thus, in order thatthe electrode make electrical contact with the silicon substrate, thedielectric film between the electrode and the silicon substrate must beremoved. This is enabled by tailoring a glass component or additives ina metal paste so that the metal paste may penetrate through thedielectric films 104, 105 to make contact with the silicon substrate,known as the “fire-through” capability.

Another important function of dielectric films 104, 105 is to restraincarrier recombination on the silicon substrate surface. Silicon atomswithin crystal are in a stable state due to a covalent bond betweenadjoining atoms. However, at the surface corresponding to the terminusof an atom array, an unstable energy level, also referred to asunsatisfied valence or dangling bond, develops because an adjoining atomto be bonded is not available. The dangling bond is electrically activeenough to capture an electric charge photo-created within siliconwhereby the charge is extinguished, thus detracting from the performanceof solar cells. To suppress the performance loss, the solar cell issubjected to a certain surface terminating treatment for reducing thedangling bond. Alternatively, the antireflection coating is givenelectric charges for substantially reducing the concentration ofelectrons or holes at the surface for thereby restraining recombinationof electrons with holes. In particular, the latter is referred to as“field effect passivation.” Silicon nitride and analogous films areknown to have positive charges and thus exert the field effectpassivation.

However, it is known that if a silicon nitride or analogous film havingpositive charges is applied to the surface of p-type silicon substrate,solar cell performance is degraded. The positive charge in the filmbiases the energy band at the p-type silicon surface toward the invertedstate, and the concentration of electrons or minority carriers becomeshigher at the silicon surface. If an electrode is formed on the p-typesilicon surface, then the electrons accumulating on the surface flow tothe electrode. Since it is the electrode on the n-type silicon side thatextracts electrons in the solar cell, the electrons flowing into thep-type silicon side electrode are lost as leak current flow from thesolar cell output. For this reason, a silicon oxide film which allegedlyhas a relatively low positive charge and an aluminum oxide film having anegative charge are now used for the passivation of p-type siliconsurface.

The following technical documents are considered to be relevant to thepresent invention.

PRIOR-ART DOCUMENTS Non-Patent Document

-   Non-Patent Document 1: S. Dauwe, L. Mittelstadt, A. Metz and R.    Hezel, Proc. the 17th European Photovoltaic Solar Energy    Conference, p. 339, 2001-   Non-Patent Document 2: J. Benik, B. Hoex, M. C. M. van de    Sanden, W. M. M. Kessels, O., Schultz and S. W. Glunz, Applied    Physics Letters, 92, 253504, 2008

SUMMARY OF INVENTION Problem to be Solved by Invention

However, the aluminum oxide film has poor fire-through capability duringelectrode formation, as compared with the silicon nitride film and thelike, so that the electric resistance between the electrode and thesilicon substrate may be increased, failing to provide satisfactorysolar cell characteristics. Then, when an electrode is formed on asilicon substrate having an aluminum oxide film formed thereon, the filmmust be configured to a pattern conformal to the electrode pattern. Thispattern configuration is generally carried out by patterning usingphotolithography or acid resist, or by etching the film with acid. Atechnique of printing an etching paste and a patterning technique basedon laser ablation are also included. These techniques, however, arescarcely acceptable in the commercial application from the standpoint ofcost because not only the number of steps is increased, but also thematerials and equipment involved are very expensive.

On the other hand, in order to maximize the passivation function of thealuminum oxide film, heat treatment around 400° C. is necessary. Thisfurther complicates the solar cell manufacturing process, becoming abarrier against cost reduction. Furthermore, common conductive pastes ofhigh-temperature cure type are difficult to fire through the aluminumoxide film, resulting in increased electric resistance and restrictingthe solar cell characteristics.

An object of the present invention, which is devised in view of theabove-discussed circumstances, is to provide a solar cell having goodfire-through capability to an aluminum oxide film, high productivity,low cost, and high efficiency, a method of manufacturing the solar cell,and a solar cell module.

Means for Solving Problem

Making extensive investigations to attain the above object, theinventors have arrived at the invention which relates to a solar cellcomprising a semiconductor substrate having a light-receiving surfaceand a non-light-receiving surface, a pn junction formed in thesemiconductor substrate, a passivation layer disposed on thelight-receiving surface and/or the non-light-receiving surface, andpower extraction electrodes disposed on the light-receiving surface andthe non-light-receiving surface. A layer including an aluminum oxidefilm having a thickness of up to 40 nm is formed as the passivationlayer, which provides for a fire-through capability during electrodeformation. A solar cell having satisfactory characteristics is thusobtainable.

Specifically, the present invention provides a solar cell, a method ofmanufacturing the same, and a solar cell module, as defined below.

Claim 1:

A solar cell comprising a semiconductor substrate having alight-receiving surface and a non-light-receiving surface, a pn junctionformed in the semiconductor substrate, a passivation layer disposed onthe light-receiving surface and/or the non-light-receiving surface, andpower extraction electrodes disposed on the light-receiving surface andthe non-light-receiving surface, wherein

said passivation layer includes an aluminum oxide film having athickness of up to 40 nm.

Claim 2:

The solar cell of claim 1 wherein said passivation layer is disposed onthe non-light-receiving surface of a p-type semiconductor substrate orthe light-receiving surface of an n-type semiconductor substrate.

Claim 3:

The solar cell of claim 1 or 2 wherein said passivation layer includesthe aluminum oxide film and another dielectric film disposed thereon,the other dielectric film being formed of silicon oxide, titanium oxide,silicon carbide or tin oxide.

Claim 4:

The solar cell of any one of claims 1 to 3 wherein said electrode is asintered product obtained by firing a conductive paste, and the sinteredproduct penetrates through the passivation layer including the aluminumoxide film so as to make electrical contact between the electrode andthe substrate.

Claim 5:

The solar cell of claim 4 wherein said sintered product contains anoxide of one or more elements selected from the group consisting of B,Na, Al, K, Ca, Si, V, Zn, Zr, Cd, Sn, Ba, Ta, Tl, Pb, and Bi.

Claim 6:

The solar cell of claim 4 or 5 wherein said aluminum oxide film has abuilt-in negative electric charge which is increased by the firing step.

Claim 7:

The solar cell of any one of claims 4 to 6 wherein a region of saidaluminum oxide film which is to be disposed immediately below theelectrode is displaced by the penetration of the sintered product, andsaid aluminum oxide film is present in at least a portion of the regionexcluding the region disposed immediately below the electrode.

Claim 8:

A solar cell module comprising a plurality of electrically connectedsolar cells as set forth in any one of claims 1 to 7.

Claim 9:

A method for manufacturing a solar cell, comprising the steps of forminga pn junction in a semiconductor substrate, forming a passivation layeron a light-receiving surface and/or a non-light-receiving surface of thesemiconductor substrate, and forming power extraction electrodes on thelight-receiving surface and the non-light-receiving surface,

wherein an aluminum oxide film having a thickness of up to 40 nm isformed as the passivation layer.

Claim 10:

The method of claim 9 wherein the electrode is formed by firing aconductive paste at 500 to 900° C. for 1 second to 30 minutes to form asintered product that penetrates through the passivation layer to makeelectrical contact between the electrode and the substrate.

Claim 11:

The method of claim 10 wherein said sintered product contains an oxideof one or more elements selected from the group consisting of B, Na, Al,K, Ca, Si, V, Zn, Zr, Cd, Sn, Ba, Ta, Tl, Pb, and Bi.

Claim 12:

The method of claim 10 or 11 wherein said aluminum oxide film has abuilt-in negative electric charge which is increased by the firing step.

Advantageous Effects of Invention

Now that an aluminum oxide film having a specific thickness is formed ona substrate surface, specifically the non-light-receiving surface of ap-type semiconductor substrate or the light-receiving surface of an-type semiconductor substrate, a satisfactory passivation function andtight electrical contact between the substrate and the electrode areobtainable merely by the step of firing a conductive paste, which is aprior art technique. The invention omits the anneal step which isnecessary in the prior art for the aluminum oxide film to exert apassivation effect, and is very effective for cost reduction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of an exemplary prior art solar cellstructure.

FIG. 2 is a cross-sectional view of one embodiment of the solar cell ofthe invention.

FIG. 3 is a cross-sectional view of another embodiment of the solar cellof the invention.

FIG. 4 is a cross-sectional view of a further embodiment of the solarcell of the invention.

FIG. 5 is a cross-sectional view of a still further embodiment of thesolar cell of the invention.

FIG. 6 is a graph showing contact resistance versus thickness ofaluminum oxide film.

FIG. 7 is a graph showing effective carrier life before and after heattreatment.

EMBODIMENTS FOR CARRYING OUT INVENTION

Some embodiments of the solar cell of the invention are described belowwith reference to the drawings although the invention is not limited tothese embodiments of the solar cell.

FIGS. 2 and 3 illustrate embodiments of the solar cell of the invention.A semiconductor substrate 201 (301) is etched with a concentratealkaline solution of sodium hydroxide or potassium hydroxide in aconcentration of 5 to 60 wt %, a mixed acid of hydrofluoric acid andnitric acid, or the like to remove saw damages on its surfaces. Thesemiconductor substrate used herein may be any of p or n-type singlecrystal silicon substrates, p or n-type polycrystalline siliconsubstrates, and p or n-type thin-film silicon substrates. The singlecrystal silicon substrate may be prepared by either the CZ method or theFZ method. For example, an as-cut single crystal {100} p-type siliconsubstrate in which high purity silicon is doped with Group III elementsuch as B, Ga or In to give a resistivity of 0.1 to 5 Ω-cm may be used.

Next, the substrate surface (light-receiving surface) is formed withmicroscopic asperities known as “texture”. Texturing is an effectivemeans for reducing the reflectance of solar cells. The texture may bereadily formed by immersing in a hot solution of alkali such as sodiumhydroxide, potassium hydroxide, potassium carbonate, sodium carbonate,sodium hydrogen carbonate or tetramethylammonium hydroxide(concentration 1 to 10 wt %, temperature 60 to 100° C.) for about 10 to30 minutes. Often, a predetermined amount of 2-propanol is dissolved inthe solution to promote the reaction.

The texturing is followed by washing with an acidic aqueous solutionsuch as hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acidor a mixture thereof. From the standpoints of cost and properties,washing in hydrochloric acid is preferred. To improve cleanness, washingmay be performed by admixing 0.5 to 5 wt % of hydrogen peroxide inhydrochloric acid solution and heating at 60 to 90° C.

To form a back surface field (BSF) layer 206 (306) on the substrate,vapor phase diffusion of boron bromide or the like is carried out at 900to 1,000° C. to form a p⁺ layer. The BSF layer may be formed on theentire back surface (depicted at 206 in FIG. 2) or locally inconformation to a pattern of back surface electrode (depicted at 306 inFIG. 3). In general silicon solar cells, the BSF layer should be formedonly on the back surface. To this end, a suitable means is preferablytaken for preventing the p⁺ layer from being formed on thelight-receiving surface, for example, by carrying out diffusion on astack of two substrates, or by forming a diffusion barrier such assilicon nitride on the light-receiving surface. Since the BSF layer inwhich impurity is diffused in a high concentration has a high carrierconcentration, the BSF layer is also effective for reducing the electricresistance between a back surface electrode 208 (308) and the substrate201 (301).

Next, vapor phase diffusion of phosphorus oxychloride is carried out toform an n-type layer 202 (302) to define a pn junction 203 (303).Typically, the pn junction must be formed only on the light-receivingsurface. To this end, a suitable means is preferably taken forpreventing phosphorus from diffusing into the back surface, for example,by carrying out diffusion on a stack of two substrates with the p⁺ layersides mated, or by forming a diffusion barrier such as silicon nitrideon the back surface. After diffusion, any glass deposit on the surfaceis removed with hydrofluoric acid or the like. Besides the vapor phasediffusion, this step may be performed by another technique of spincoating, spraying or otherwise applying a diffusing agent.

Next, a dielectric film 204 (304) is formed which serves as anantireflective film on the substrate surface or light-receiving surface.As the dielectric film, for example, silicon nitride is deposited to athickness of about 50 to 100 nm. For deposition, a chemical vapordeposition (abbreviated as CVD, hereinafter) system is used. A mixtureof monosilane (SiH₄) and ammonia (NH₃) is often used as the reactantgas. In some cases, nitrogen may be used instead of NH₃. The desiredrefractive index may be accomplished by diluting the depositing species,adjusting the process pressure, and diluting the reactant gas, with H₂gas. The dielectric film is not limited to a silicon nitride film, and afilm of silicon oxide, silicon carbide or titanium oxide which is formedby heat treatment, atomic layer deposition (abbreviated as ALD,hereinafter) or the like may be used instead.

On the other hand, a passivation film or layer 205 (305) including analuminum oxide film 205 a (305 a) is formed on the back surface orp-type silicon surface. While the CVD or ALD method is often used forthe deposition of aluminum oxide film, vacuum evaporation or sputteringmay also be used herein. The CVD and ALD methods typically usetrimethylaluminum (TMA) as the reactant, and hydrogen (H₂) or argon (Ar)as the carrier gas. Oxygen (O₂), carbon dioxide (CO₂), water (H₂O),ozone (O₃) or the like is used as the oxidizing agent for aluminum. Oneexemplary reaction scheme is as follows.

Al(CH₃)₃+1.5H₂O→0.5Al₂O₃+3CH₄

Film deposition by the CVD method proceeds while these molecules aredecomposed and deposited on the substrate. This decomposition may bethermally induced at 100 to 400° C. by heating the substrate, orelectromagnetically induced at 100 to 400° C. by applying ahigh-frequency electric field. A crystalline or amorphous film havingany arbitrary constitutional ratio of aluminum to oxygen may be formed.

The aluminum oxide film thus obtained bears a negative electric charge,which is believed to be derived from the following chemical reactionscheme. Herein, for simplicity sake, reference is made to reaction inAl₂O₃ film.

2Al₂O₃→3(AlO_(4/2))¹⁻+Al³⁺

The film is electrically neutral as such. As Al³⁺ combines with oxygenin the aluminum oxide film to form a donor/acceptor pair, by which thepositive charge is extinct, the film eventually bears a negative charge.

It is believed that the negative charge-generating mechanism describedabove is equally applicable to other systems such as an aluminum oxidefilm deviating from the stoichiometry, i.e., of Al_(1-x)O_(x) wherein xis an arbitrary constant, or a mixture of aluminum oxide with hydrogen,carbon, nitrogen or the like. Namely, a negative charge may be generatedwhen the chemical scheme stands between Al and O, at least in part, in asystem where Al and O are co-present.

Continuing empirical studies on the thickness of aluminum oxide film,the inventors have found that the film thickness is up to 40 nm,preferably up to 30 nm, and more preferably up to 20 nm. Although thelower limit is not critical, the film thickness is typically at least 1nm to provide uniform coverage over the substrate surface.

To further enhance the optical confinement effect at the back surface,another dielectric film 205 b (305 b) may be formed on the aluminumoxide film 205 a (305 a) as an overlay. For the dielectric film 205 b(305 b), it is preferable from the optical aspect to use silicon oxide(SiO, SiO₂), but also acceptable to use titanium oxide (TiO, 1100,silicon carbide (SiC), tin oxide (SnO, SnO₂, SnO₃) or the like. Thedielectric film 205 b (305 b) on the back surface preferably has athickness of 50 to 250 nm, more preferably 100 to 200 nm. If the film istoo thin or thick, the optical confinement effect may becomeinsufficient.

Next, electrodes 207 and 208 (307 and 308) are formed on thelight-receiving surface and the non-light-receiving surface (backsurface) of the substrate, respectively. The electrodes are formed byprinting a conductive paste, typically a silver paste obtained by mixinga silver powder and glass frit with an organic binder, on thelight-receiving surface and the back surface, and firing the paste at atemperature of about 500 to 900° C., preferably about 700 to 850° C.,for 1 second to 30 minutes, preferably 3 seconds to 15 minutes. The heattreatment causes the passivation film to be attacked by the conductivepaste, typically silver paste, whereby the electrode in the form of asintered product of the conductive paste fires or penetrates through thepassivation film to make electrical contact with the silicon substrate.Notably, firing of the electrodes on the light-receiving surface and theback surface may be carried out separately on each surface.

It is the metal oxide in the conductive paste that provides theconductive paste with a passivation film fire-through capability. Themetal oxide used herein may be an oxide of one or more elements selectedfrom the group consisting of B, Na, Al, K, Ca, Si, V, Zn, Zr, Cd, Sn,Ba, Ta, Tl, Pb, and Bi. In order that firing cause the paste topenetrate through the aluminum oxide film and optional dielectric filmto make good contact with the substrate, glass materials such as B—Pb—O,B—Pb—Zn—O, B—Zn—V—O, B—Si—Pb—O, B—Si—Pb—Al—O, B—Si—Bi—Pb—O, andB—Si—Zn—O base materials may be used.

A region of the aluminum oxide film which is to be disposed immediatelybelow the electrode is displaced by the penetration of the sinteredproduct, and the aluminum oxide film is formed in at least a portion ofthe region excluding the region disposed immediately below theelectrode. For gaining a satisfactory passivation effect, the aluminumoxide film is preferably formed on the entire non-light-receivingsurface (back surface) and/or the entire light-receiving surfaceexcluding the region disposed immediately below the electrode,specifically on the entire non-light-receiving surface of p-type siliconsubstrate or the entire light-receiving surface of n-type siliconsubstrate.

Although the embodiments of the solar cell using a p-type siliconsubstrate have been described, the invention is applicable to a solarcell using an n-type silicon substrate. As shown in FIGS. 4 and 5, ann-type silicon substrate 401 (501) is prepared by doping high-puritysilicon with Group V element such as P, As or Sb, and typically adjustedto a resistivity of 0.1 to 5 Ω-cm. The n-type silicon solar cell may bemanufactured by the same method as the p-type silicon solar cell, exceptthat it is essential to form a p⁺ layer 402 (502) in order to form a pnjunction 403 (503). On the other hand, an n⁺ layer for forming a BSFlayer on the back surface may be formed on the entire back surface(depicted at 406 in FIG. 4) or formed locally in conformation to apattern of back surface electrode (depicted at 506 in FIG. 5).

The light-receiving surface may be passivated by forming an aluminumoxide film 405 a (505 a) on the surface of a p+ layer 402 (502)according to the invention, and forming another dielectric film 405 b(505 b), typically a dielectric film of silicon oxide (SiO, SiO₂),titanium oxide (TiO, TiO₂, silicon carbide (SiC) or tin oxide (SnO,SnO₂, SnO₃) thereon as an overlay. On the n layer on the back surface, adielectric film 404 (504) of silicon nitride, silicon oxide, siliconcarbide, titanium oxide or the like is preferably formed. The filmforming conditions including film thickness and the conditions forforming electrodes 407 and 408 (507 and 508) may be the same as thosefor the p-type silicon substrate.

On the solar cell back surface, a reflector is preferably provided forreflecting back the light transmitted by the substrate. As thereflector, aluminum or silver which can be formed as a film by vacuumevaporation or the like may be used. Equivalent effect may be achieved,without additional treatment, merely by applying a white back sheet to asolar cell module. On the other hand, the reflector may be omitted, andelectricity can be generated by causing scattering light enter the backsurface. Furthermore, electricity can be generated by setting the solarcell such that the back surface may become the light-receiving surfaceside.

According to the invention, a solar cell module is obtained byelectrically connecting a plurality of solar cells manufactured asabove.

EXAMPLE

Experiments, Examples and Comparative Examples are given below forfurther illustrating the invention although the invention is not limitedto the Examples.

Experiment 1 Investigation of Electrode Contact Resistance

To investigate the thickness of an aluminum oxide film, first aconductive paste commonly used for the fire-through capability tosilicon oxide film was used to examine its fire-through capability toaluminum oxide film. The fire-through capability can be evaluated interms of contact resistance between the electrode and the siliconsubstrate.

Vapor phase diffusion of boron bromide was carried out on a texturedp-type silicon wafer of 15 cm squares having a thickness of 240 μm,thereby diffusing boron therein to form a p⁺ layer. An aluminum oxidefilm was formed on the p⁺ layer by the ALD method, and a silicon oxidefilm was formed thereon by the plasma CVD method. The thickness of thesilicon oxide film was adjusted such that the total thickness ofaluminum oxide film and silicon oxide film was 100 nm. On thesepassivation films, a commercially available fire-through capabilitysilver paste was printed in a comb-shaped pattern, and fired in a rapidthermal processing (RTP) furnace at a peak temperature of 800° C. for 3seconds. The number of samples prepared was 5 samples for each set ofconditions.

To evaluate contact resistance by the ladder method, strip-shapedspecimens of 1 cm wide and 5 cm long were cut out of a wafer at 5positions and measured.

FIG. 6 diagrammatically illustrates contact resistance versus thicknessof aluminum oxide film. For the combination of silicon oxide film andaluminum oxide film, when the aluminum oxide film thickness is reducedto about 40 nm, the contact resistance shows a drastic drop, and whenthe aluminum oxide film thickness is reduced to 20 nm or less, thecontact resistance reaches a value approximate to that of silicon oxidefilm of 100 nm thick (aluminum oxide film thickness=0 nm). From theseresults, the aluminum oxide film thickness that provides forsatisfactory electrical contact is judged to be 40 nm or less,preferably 30 nm or less, and more preferably 20 nm or less.

Experiment 2 Investigation of Passivation Effect Upon Electrode Firing

Next, a test of measuring carrier life was carried out to examine thepassivation effect versus thickness of aluminum oxide film.

On opposite surfaces of a p-type silicon wafer of 15 cm square having athickness of 200 μm which had been mirror finished by acid etching,aluminum oxide films of varying thickness were formed by the ALD method.To impart the thermal hysteresis of electrode firing heat treatment,each sample was heat treated in a RTP furnace at a peak temperature of800° C. for 3 seconds.

FIG. 7 diagrammatically illustrates the measurement results of effectivecarrier life before and after heat treatment. The effective carrier lifeis an overall carrier life including a carrier life in crystal bulksilicon and a carrier life at the silicon-aluminum oxide film interface,expressed in microsecond unit. In FIG. 7, the broken line with blacksquares designates the effective carrier life before heat treatment andthe broken line with white squares designates the effective carrier lifeafter heat treatment.

For all the samples, a phenomenon that the carrier life was extended byheat treatment was observed, and the results indicated that the value ofcarrier life does not depend on the thickness of aluminum oxide film. Itwas confirmed by CV measurement that the extension of the carrier lifeby heat treatment is attributable to an increase of built-in negativecharge quantity in aluminum oxide film by heat treatment. The chargequantity before heat treatment was 1×10¹° to 3×10¹⁰ C-cm⁻² whereas thecharge quantity after heat treatment increased to about 3×10¹² C-cm⁻²for all varying thickness samples. Based on the fact that thepassivation effect of aluminum oxide film does not depend on itsthickness, it is believed that charges within the film collect near theinterface between silicon substrate and aluminum oxide film.

It is evident from these results that satisfactory passivation effect isachievable even when the thickness of aluminum oxide film is reduced to40 nm or less. It has been newly found that a high negative chargequantity of aluminum oxide film is fully developed by brief heattreatment during electrode firing, and the low-temperature anneal stepwhich has been a problem can be omitted.

Example 1

One hundred (100) as-cut, boron-doped, {100} p-type silicon substrateshaving a thickness of 250 μm and a resistivity of 1 Ω-cm were treatedwith a hot conc. potassium hydroxide aqueous solution for saw damageremoval, immersed in a potassium hydroxide/2-propanol aqueous solutionfor texturing, and subsequently washed with a hydrochloric acid/hydrogenperoxide mixed solution. Next, a stack of substrates with theirlight-receiving surfaces mated was heat treated at 1000° C. in a boronbromide atmosphere to form a p⁺ layer. Subsequently, a stack ofsubstrates with their back surfaces mated was heat treated at 850° C. ina phosphorus oxychloride atmosphere to form a pn junction. Afterdiffusion, the glass layer was removed with hydrofluoric acid, and thesubstrates were washed with deionized water and dried. After thesetreatments, a silicon nitride film of 100 nm thick was deposited as anantireflective film on the entire light-receiving surface in a plasmaCVD system.

On half (50 substrates) of the thus treated substrates, a back sidepassivation film was formed. On these 50 substrates, an aluminum oxidefilm of 20 nm thick was deposited on the entire back surface at asubstrate temperature of 200° C. in an ALD system, using TMA as thereactant gas and oxygen as the oxidizing agent. The aluminum oxide filmresulting from this process was stoichiometric amorphous Al₂O₃.Thereafter, a silicon oxide film of 150 nm was deposited in a sputteringsystem.

Next, Ag paste was screen printed in a comb-shaped pattern on thelight-receiving surface and back surface of all the substrates anddried. Then the paste was fired in air at 800° C. for 3 seconds wherebyAg electrodes penetrated through the dielectric films on both thelight-receiving surface and the back surface to make electric conductionto the silicon substrate. In a vacuum evaporation system, an Al film of2 μm thick was formed as a reflector on the back side of the solar cell.

Comparative Example 1

The remaining 50 substrates prepared in Example 1 were processed as inExample 1 except that a silicon nitride film of 100 nm thick wasdeposited on the back surface by the same method as applied to thesubstrate light-receiving surface in Example 1.

The solar cells in Example 1 and Comparative Example 1 were measured forcharacteristics by a current-voltage tester using simulator solar lightwith air mass 1.5. The results are reported in Table 1, indicating thatthe performance of solar cells in Example 1 as the practice of theinvention is superior to the performance of solar cells in ComparativeExample 1.

TABLE 1 Dielectric Short- Open- film on non- circuit circuit FillConversion light-receiving current, voltage, factor, efficiency, surfacemA/cm² V % % Example 1 Al₂O₃ 20 nm + 37.8 0.641 78.4 19.0 SiO₂ 150 nmComparative SiN 100 nm 36.0 0.636 77.5 17.7 Example 1

In Example 1 within the scope of the invention, satisfactory electriccontact is obtainable even though the thickness of dielectric film onthe back surface is greater than that of Comparative Example 1. Inaddition, since the leak current is eliminated due to the absence of aninversion layer, a satisfactory fill factor is obtained, and both theopen-circuit voltage and short-circuit current are significantlyimproved.

Example 2

One hundred (100) as-cut, phosphorus-doped, {100} n-type siliconsubstrates having a thickness of 250 μm and a resistivity of 1 Ω-cm weretreated with a hot conc. potassium hydroxide aqueous solution for sawdamage removal, immersed in a potassium hydroxide/2-propanol aqueoussolution for texturing, and subsequently washed with a hydrochloricacid/hydrogen peroxide mixed solution. Next, a stack of substrates withtheir back surfaces mated was heat treated at 1000° C. in a boronbromide atmosphere to form a pn junction. Subsequently, a stack ofsubstrates with their light-receiving surfaces mated was heat treated at850° C. in a phosphorus oxychloride atmosphere to form a BSF layer.After diffusion, the glass layer was removed with hydrofluoric acid, andthe substrates were washed with deionized water and dried. After thesetreatments, a silicon nitride film of 100 nm thick was deposited as aback side dielectric film on the entire back surface in a plasma CVDsystem.

On half (50 substrates) of the thus treated substrates, alight-receiving surface passivation film was formed. On these 50substrates, an aluminum oxide film of 20 nm thick was deposited on theentire light-receiving surface at a substrate temperature of 200° C. inan ALD system, using TMA as the reactant gas and oxygen as the oxidizingagent. The aluminum oxide film resulting from this process wasstoichiometric amorphous Al₂O₃. Thereafter, a titanium oxide film of 50nm was deposited by the atmospheric CVD method.

Next, Ag paste was screen printed in a comb-shaped pattern on thelight-receiving surface and back surface of all the substrates anddried. Then the paste was fired in air at 800° C. for 3 seconds wherebyAg electrodes penetrated through the dielectric films on both thelight-receiving surface and the back surface to make electric conductionto the silicon substrate. In a vacuum evaporation system, an Al film of2 μm thick was formed as a reflector on the back side of the solar cell.

Comparative Example 2

The remaining 50 substrates prepared in Example 2 were processed as inExample 2 except that a silicon nitride film of 100 nm thick wasdeposited on the light-receiving surface by the same method as appliedto the substrate back surface in Example 2.

The solar cells in Example 2 and Comparative Example 2 were measured forcharacteristics by a current-voltage tester using simulator solar lightwith air mass 1.5. The results are reported in Table 2, indicating thatthe performance of solar cells in Example 2 as the practice of theinvention is superior to the performance of solar cells in ComparativeExample 2.

TABLE 2 Dielectric Short- Open- film on circuit circuit Fill Conversionlight-receiving current, voltage, factor, efficiency, surface mA/cm² V %% Example 2 Al₂O₃ 20 nm + 36.5 0.651 78.8 18.7 TiO 50 nm Comparative SiN100 nm 36.2 0.637 78.0 18.0 Example 2

DESCRIPTION OF REFERENCE NUMERALS

-   101, 201, 301, 401, 501: semiconductor substrate-   102, 202, 302: n-type layer-   402, 502: p-type layer-   103, 203, 303, 403, 503: pn junction-   104, 105, 204, 304, 404, 504: dielectric film-   205, 305, 405, 505: passivation film-   205 a, 305 a, 405 a, 505 a: aluminum oxide film-   205 b, 305 b, 405 b, 505 b: dielectric film-   206, 306, 406, 506: back surface field (BSF) layer-   106, 207, 307, 407, 507: light-receiving surface electrode-   107, 208, 308, 408, 508: back surface electrode

1. A solar cell comprising a semiconductor substrate having alight-receiving surface and a non-light-receiving surface, a pn junctionformed in the semiconductor substrate, a passivation layer disposed onthe light-receiving surface and/or the non-light-receiving surface, andpower extraction electrodes disposed on the light-receiving surface andthe non-light-receiving surface, wherein said passivation layer includesan aluminum oxide film having a thickness of up to 40 nm.
 2. The solarcell of claim 1 wherein said passivation layer is disposed on thenon-light-receiving surface of a p-type semiconductor substrate or thelight-receiving surface of an n-type semiconductor substrate.
 3. Thesolar cell of claim 1 wherein said passivation layer includes thealuminum oxide film and another dielectric film disposed thereon, theother dielectric film being formed of silicon oxide, titanium oxide,silicon carbide or tin oxide.
 4. The solar cell of claim 1 wherein saidelectrode is a sintered product obtained by firing a conductive paste,and the sintered product penetrates through the passivation layerincluding the aluminum oxide film so as to make electrical contactbetween the electrode and the substrate.
 5. The solar cell of claim 4wherein said sintered product contains an oxide of one or more elementsselected from the group consisting of B, Na, Al, K, Ca, Si, V, Zn, Zr,Cd, Sn, Ba, Ta, Tl, Pb, and Bi.
 6. The solar cell of claim 4 whereinsaid aluminum oxide film has a built-in negative electric charge whichis increased by the firing step.
 7. The solar cell claim 4 wherein aregion of said aluminum oxide film which is to be disposed immediatelybelow the electrode is displaced by the penetration of the sinteredproduct, and said aluminum oxide film is present in at least a portionof the region excluding the region disposed immediately below theelectrode.
 8. A solar cell module comprising a plurality of electricallyconnected solar cells as set forth in claim
 1. 9. A method formanufacturing a solar cell, comprising the steps of forming a pnjunction in a semiconductor substrate, forming a passivation layer on alight-receiving surface and/or a non-light-receiving surface of thesemiconductor substrate, and forming power extraction electrodes on thelight-receiving surface and the non-light-receiving surface, wherein analuminum oxide film having a thickness of up to 40 nm is formed as thepassivation layer.
 10. The method of claim 9 wherein the electrode isformed by firing a conductive paste at 500 to 900° C. for 1 second to 30minutes to form a sintered product that penetrates through thepassivation layer to make electrical contact between the electrode andthe substrate.
 11. The method of claim 10 wherein said sintered productcontains an oxide of one or more elements selected from the groupconsisting of B, Na, Al, K, Ca, Si, V, Zn, Zr, Cd, Sn, Ba, Ta, Tl, Pb,and Bi.
 12. The method of claim 10 wherein said aluminum oxide film hasa built-in negative electric charge which is increased by the firingstep.